Has my beer stopped fermenting?

This is a common question that crops up amongst new brewers waiting expectantly on their first or second batch of beer. Fortunately it’s an easy question to answer -and a good opportunity to learn what happens during fermentation as well as a bit about using hydrometers. Read on!

Firstly it’s a good idea to know what to expect of a fermenting batch of beer. Most of us know that there should be some vigorous bubbling from the airlock (much to the amusement of family members), and a thick head of yeast on top of the beer. This will slow down and eventually subside after a few days, signifying that the time for bottling is soon approaching.

But what is really going on under that lid?

To understand what’s really happening in the fermenter we need to understand the basics of what our yeast get up to. These friendly fungi are the ones actually making our beer for us at this point, so their habits and happiness is worth understanding.

When pitched to the fermenter, the yeast first acclimatise to their new environment and begin to multiply many times over. The yeast use oxygen during this reproductive phase and this is the reason that brewers shake the fermenter vigorously for several minutes to oxygenate the wort before pitching the yeast. The yeast do not yet make any alcohol or carbon dioxide at this early stage -they are far too busy populating the contents of the fermenter! This quiet start is referred to as the lag phase and is where we expectantly wait for 12-24 hours for the yeast population to grow, and then begin on the important (and rowdy) task of producing alcohol!

Well that’s pretty simple- but you guessed correctly that if the wort is not oxygenated the yeast won’t be able to multiply. This can happen if a brewer forgets to oxygenate or doesn’t shake the fermenter quite enough ( 4-5 minutes is best) . Another important factor is just how much yeast is pitched. A left over half sachet from a few months ago is not going to get the job done! There need to be enough healthy, viable yeast to get off to a strong start populating the wort. Too few simply cannot multiply enough times. So...always pitch a full rehydrated yeast sachet. The lag phase will be short and the yeast happy and plentiful!

The next phase is the vigorous conversion of sugars to alcohol and carbon dioxide. The yeast have run out of oxygen and now turn to sugar for sustenance. They can survive without oxygen and enter a new phase known as ‘anaerobic’ (without oxygen). The yeast now produce alcohol, flavour compounds called esters and phenols, and work their way through the sugar. The carbon dioxide simultaneously produces a large head of yeasty froth on top of the beer and signifies the peak of fermentation. This busy and productive time is also commonly called ‘primary fermentation’ and is essentially when the magic happens and the young beer is created.

Once the primary fermentation has begun in earnest there is not too much that can dissuade the yeast from quitting-excepting really cold conditions- below 16C say. As a general rule the cooler temperatures result in slower (and perhaps less energetic) primary fermentation times and warmer faster. Often this stage is over very quickly- two to three days is not uncommon with 4-7 being average. But it’s not quite over yet...

The last phase of fermentation is where our original question usually arises. Is the beer almost ready? -What is it doing now?-Should I bottle it this weekend? I’m getting thirsty!

After the initial crescendo of primary fermentation, the beer can look like it has completed its job. This is not at all true though. The young beer has now entered its last important phase known as secondary fermentation. At this point the yeast are still consuming any remaining sugars- though at a much slower rate – and also consuming by-products of the primary phase. The yeast is finishing the job thoroughly and also cleaning up after itself! With the sugar almost gone, the yeast finds and breaks down various other compounds which later affect the finished flavour of the beer. You could see it a gradual finishing or pre- maturation phase.

Once the yeast has exhausted its supply of food it begins to go dormant. It clumps together and drops to the bottom of the fermenter, eventually leaving the beer clear(a process called flocculation) English brewers refer to this as the beer ’dropping bright’ .Depending on the yeast this can happen quite quickly or sometimes take a while.

The fermentation is essentially now over, and the beer is beginning to mature. So is my beer ready? Typically, yes, but let’s look at some important time frames and scenarios. Being aware of variables is what is important now...

Most advice to home brewers suggests a total fermentation time of 12-14 days. This is assuming a ‘textbook brew’ with plenty of healthy yeast and fermentation at a suitable temperature ( 17C-24C ). Generally things work out fine within this timeframe and temperature range. The beer is then bottled, undergoes carbonation from a small secondary fermentation (from added priming sugar) and then has a week or three to mature before drinking.

Now that you have a general idea of what’s going on under the lid let’s quickly look at how a hydrometer can help us measure the yeast’s progress, and help calculate the alcohol content of our finished beer. A hydrometer is a useful floating measure that will sink lower or float higher depending on how much dissolved sugar is in our beer. Most brewers take a gravity reading just before the yeast is pitched, and then again before bottling. As the yeast consumes the sugar, the hydrometer readings will gradually drop (as the hydrometer floats lower). Generally, taking a reading during fermentation is quite unnecessary if things are progressing normally. Remember, frequently opening your fermenter exposes the contents to bacteria and wild yeast, though if you have a side tap this is a bit less of an issue. There is a good bit of common advice that if one takes a reading for a few consecutive days and gets the same results, the yeast has finished the work –though again this typically this isn’t necessary unless you really aren’t sure.

Measuring the alcohol content is done by taking (and writing down!) a gravity reading just prior to fermentation called the Original Gravity (shortened to O.G.), and then, at the end of fermentation the Final Gravity (F.G.). The F.G.is subtracted from the O.G. and multiplied by 0.129 to give the Alcohol by Volume (the same %ABV we see on our commercial beer and wine bottles). Here are three examples of some typical strengths of beer, beginning with an average strength.

In another post I will take a more detailed look at using a hydrometer, but that’s a useful start for now!

To round up, let’s quickly look at where a fermentation cycle can go astray, take too long or just seem wrong – and why. By now you now have a pretty clear idea of the fermentation phases happening inside fermenter as well as some of the possible pitfalls. Here’s a set of easy ‘Best case’ versus ‘Concerned! ‘scenarios to help you!

The lag phase: Best case scenario:

A full sachet of rehydrated yeast s pitched and begins to ferment (bubbling airlock) after 6-12 hours (or even sooner). The temperature is within target range specified on sachet (or manufacturer’s website)The wort was vigorously shaken and splashed to aerate it thoroughly. A light froth begins to form on the beer.

The lag phase. Concerned! :

After 24 hours nothing is happening or there is only very occasional bubbling from the airlock.

Check that the airlock is properly secured- often a slight leak in the airlock’s grommet seal stops it from bubbling as the CO2 is escaping around it. 17C to 24C is your ballpark. A quick peak in the fermenter may reveal an actively fermenting beer!

Is the fermenter sitting in an icy winter garage or is the temperature really low? Keep the fermenter in a warm enough room. 17C to 24C is your ballpark. Did you aerate enough (or possibly forget)? If not, do so immediately- it should help get things going.

Under-pitching(not enough) yeast will also slow things down considerably. Consider pitching more yeast. Slow bubbling does mean something is happening – often it will simply get going properly in another few hours. If you have checked through the variables, grab a beer and don’t worry.

Primary Fermentation: Best case scenario:

A rocky head of yeast forms on the beer and the airlock is happily bubbling away. Regular bubbling slows right down after three to six days (sometimes sooner) . A foam line can be visible from the high ‘krausen’ on some fermenters. Remember: warmer temperatures result in faster fermentations than colder.

Primary fermentation. Concerned!:

Very slow or sluggish fermentation.

Cold temperatures are your main cause for concern here if the other factors like yeast and aeration have been checked. Move the fermenter to a warmer area.

Warm temperature and a low gravity beer can lead to very quick fermentations- as short as 2-3 days. Primary fermentation may have already taken place.

Secondary fermentation. Best case scenario:

The airlock slows down to a very occasional bubble. The surface of the beer clears with a few light patches of thin foam here and there. Many brewers use the airlock as an indicator at this point-once activity stops completely the beer is done. This works pretty well, just be aware that a very cold spell can also make your yeast go temporarily dormant!

At this stage the beer begins to clear of yeast, and after a few days the yeast should have formed a thick visible layer at the bottom and the beer cleared. At this time you can bottle within a few days or let it mature for another week or so if you do not have time. Remember; two to three weeks are the best length of time to wait before bottling.

Secondary fermentation. Concerned! :

The beer smells ‘off’ or the airlock is persistently bubbling.

A bad smell (and there are many kinds!) or a visible growth on top of the beer means that wild yeast and/or bacteria have got in and wreaked havoc. Pay more attention to sanitizing. It’s not common, but it does sometimes happen-quite often more in summer or autumn when there is a lot of wild yeast floating around. A ‘green apple’ smell is normal for young beer, and a light sulphur smell is also known with some yeast strains.

A persistent fermentation can mean two things. Most probably the yeast is simply taking longer to get the job done or, less likely, wild yeast and bacteria have taken residence. Most likely it is the strain of yeast that is simply taking a little longer than usual. If in doubt have a sniff in the fermenter. It should smell initially of carbon dioxide followed by a clean beer/hoppy/slight green apple aroma. If in doubt, a gravity reading will help.

With a little experience and the information here, you will soon always know when your beer is ready. And it’s a very satisfying thing to know too-good luck with your brews!

This article is copyright and used with exclusive permission to Beerlab. Nick Birkby 2013